A cable tester (CT) can test the cable length, cable bandwidth, and other characteristics of a cable or transmission line that conducts a signal from a physical layer device (PHY) to a remote terminal or termination. The CT may be located in or near the PHY. The cable length can vary from zero to several hundred meters or more.
The cable can attenuate the AC and DC components of the signal, distort the signal, and introduce other impairments, such as impairments that are a function of cable length. For example, the cable can attenuate a DC component that powers the remote terminal. The AC components of the signal may be subject to distortion, such as dispersion and inter-symbol interference (ISI), cross-talk from nearby conductors in the cable, and other impairments.
The cable may be a multi-conductor cable, such as a CAT 5 or CAT 5e cable that includes multiple unshielded twisted pairs, a parallel data cable or ribbon cable, a single conductor pair, such as a coaxial cable a single twisted pair, and the like. Both types of cable can carry an injected signal or source waveform from a PHY to a remote terminal and a reflected signal back to the PHY. A multi-conductor cable can carry multiple signals, such as the signal from the PHY to the remote terminal and other signals on other conductors, such as a loop-back signal from the remote signal back to the PHY.
The generator impedance of the PHY or the termination impedance of the remote terminal may be mismatched to the cable impedance. An impedance-mismatched generator or termination can reflect a portion of the injected signal back to the PHY. For example, an imperfectly impedance matched termination can cause a far-end reflection that returns an echo to the signal source or generator in the PHY after a round-trip propagation delay. When the velocity of the injected signal on the cable is known, then the distance between the PHY and the remote terminal can be determined by measuring the round-trip propagation delay by the signal group velocity.
The time domain reflectometry (TDR) method is a radar-like method that can determine the length of the cable between a PHY and a remote terminal. In the TDR method, the CT can generate a pulse with fast rise and fall times compared to a data symbol clock period then measure the reflected signal from the cable. The reflections may be near-end reflections, intermediate-distance reflections, or far-end reflections from impedance discontinuities, such as opens or shorts, at corresponding distances from the CT to the remote terminal or cable termination. The TDR method can work well, up to about 400 meters, when the impedance mismatch of the remote terminal to the cable is large, such as an open-circuit or a short-circuit termination. However, the TDR method may miss small reflections from a mis-matched termination, especially when the reflections are masked by noise, such a receiver noise, cross-talk on the cable, quantization noise from an analog to digital converter (ADC), and the like.